1 files changed, 109 insertions, 0 deletions
diff --git a/src/net/smtp/auth.go b/src/net/smtp/auth.go
new file mode 100644
index 0000000..6d461ac
--- /dev/null
+++ b/src/net/smtp/auth.go
@@ -0,0 +1,109 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package smtp
+
+import (
+	"crypto/hmac"
+	"crypto/md5"
+	"errors"
+	"fmt"
+)
+
+// Auth is implemented by an SMTP authentication mechanism.
+type Auth interface {
+	// Start begins an authentication with a server.
+	// It returns the name of the authentication protocol
+	// and optionally data to include in the initial AUTH message
+	// sent to the server.
+	// If it returns a non-nil error, the SMTP client aborts
+	// the authentication attempt and closes the connection.
+	Start(server *ServerInfo) (proto string, toServer []byte, err error)
+
+	// Next continues the authentication. The server has just sent
+	// the fromServer data. If more is true, the server expects a
+	// response, which Next should return as toServer; otherwise
+	// Next should return toServer == nil.
+	// If Next returns a non-nil error, the SMTP client aborts
+	// the authentication attempt and closes the connection.
+	Next(fromServer []byte, more bool) (toServer []byte, err error)
+}
+
+// ServerInfo records information about an SMTP server.
+type ServerInfo struct {
+	Name string   // SMTP server name
+	TLS  bool     // using TLS, with valid certificate for Name
+	Auth []string // advertised authentication mechanisms
+}
+
+type plainAuth struct {
+	identity, username, password string
+	host                         string
+}
+
+// PlainAuth returns an [Auth] that implements the PLAIN authentication
+// mechanism as defined in RFC 4616. The returned Auth uses the given
+// username and password to authenticate to host and act as identity.
+// Usually identity should be the empty string, to act as username.
+//
+// PlainAuth will only send the credentials if the connection is using TLS
+// or is connected to localhost. Otherwise authentication will fail with an
+// error, without sending the credentials.
+func PlainAuth(identity, username, password, host string) Auth {
+	return &plainAuth{identity, username, password, host}
+}
+
+func isLocalhost(name string) bool {
+	return name == "localhost" || name == "127.0.0.1" || name == "::1"
+}
+
+func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
+	// Must have TLS, or else localhost server.
+	// Note: If TLS is not true, then we can't trust ANYTHING in ServerInfo.
+	// In particular, it doesn't matter if the server advertises PLAIN auth.
+	// That might just be the attacker saying
+	// "it's ok, you can trust me with your password."
+	if !server.TLS && !isLocalhost(server.Name) {
+		return "", nil, errors.New("unencrypted connection")
+	}
+	if server.Name != a.host {
+		return "", nil, errors.New("wrong host name")
+	}
+	resp := []byte(a.identity + "\x00" + a.username + "\x00" + a.password)
+	return "PLAIN", resp, nil
+}
+
+func (a *plainAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		// We've already sent everything.
+		return nil, errors.New("unexpected server challenge")
+	}
+	return nil, nil
+}
+
+type cramMD5Auth struct {
+	username, secret string
+}
+
+// CRAMMD5Auth returns an [Auth] that implements the CRAM-MD5 authentication
+// mechanism as defined in RFC 2195.
+// The returned Auth uses the given username and secret to authenticate
+// to the server using the challenge-response mechanism.
+func CRAMMD5Auth(username, secret string) Auth {
+	return &cramMD5Auth{username, secret}
+}
+
+func (a *cramMD5Auth) Start(server *ServerInfo) (string, []byte, error) {
+	return "CRAM-MD5", nil, nil
+}
+
+func (a *cramMD5Auth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		d := hmac.New(md5.New, []byte(a.secret))
+		d.Write(fromServer)
+		s := make([]byte, 0, d.Size())
+		return fmt.Appendf(nil, "%s %x", a.username, d.Sum(s)), nil
+	}
+	return nil, nil
+}